Why does Blender slow down when I duplicate a model repeatedly?
Blender's viewport is a more or less simplified 2D representation of what your 3D scene looks like from a certain point of view.

This representation needs to get updated in real-time (if possible) if you rotate a scene and that update needs to be calculated by your GPU as well as your CPU. 

As Shubol3D already hinted, duplicating a model also duplicates geometry. Even further, if you have assigned materials and modifiers, they get duplicated as well. 

Mathematically every vertex in your scene is described by three vectors, x,y and z. But there is more data behind a vertex than just the geometrical coordinates. Color and light values are only two of them. Imagine a long list of those values in a spread sheet. The more geometry, the longer the spread sheet, the more time it takes for your hardware to update the information that needs to be changed to display your scene.

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